HFSE-RICH CLINOPYROXENE INCLUSION AND ITS IMPLICATION FOR THE GENESIS OF DIAMONDIFEROUS ZIRCON IN LULIANGSHAN UHP GARNET PERIDOTITES: A PARGASITE BREAKDOWN MODEL

Crystallization of ultra-high pressure metamorphic zircon in peridotite is enigmatic because it requires high Zr concentration in the ultramafic host rock, which is extremely low in incompatible elements. We use clinopyroxene in peridotite as an indicator of PT conditions and Zr concentrations in different metamorphic stages, to evaluate the formation of UHP zircon. The studied clinopyroxens are from three different metamorphic regimes (inclusions in garnets, matrix and kelyphite after garnets) of the Luliangshan UHP garnet peridotite, a diamondiferous zircon-bearing peridotite from North Qaidam, northwestern China. The clinopyroxene inclusions in garnets have the highest HFSE concentrations and depleted HREE (e.g. chondrite normalized Yb_N=0.41-1.43, La_N/Yb_N=20-60). They have primitive mantle normalized Nb_N of 0.7-52 (avg. 17.4) and Zr=21-36 ppm (avg. 27.9 ppm). Kelyphitic clinopyroxenes after garnets have high HFSE (Nb_N=1-10, and Zr=26-41 ppm, avg. 31.9 ppm) and high HREE (e.g. Yb_N=4-23, La_N/Yb_N=1.6-7). The matrix clinopyroxenes have the lowest concentrations of trace elements, especially of HFSE (Nb_N, Ta_N<0.6, Zr=4.4-11.8 ppm, avg. 7.8 ppm), which are concordant with the arc-type cumulates origin of the protolith. Based on partition coefficients, the melt in equilibrium with inclusion clinopyroxene has positive Zr anomaly (Zr_N/Zr*_N=1.24), and the melt in equilibrium with matrix clinopyroxene has negative Zr anomaly (Zr_N/Zr*_N=0.21-0.42).

Petrological observations and experimental constraints suggest that the following reactions may account for the enrichment of HFSE in the clinopyroxene inclusions in garnet and the formation of diamond bearing zircon:

Pargasite = 0.3 cpx + 0.6 melt + miner (ol + spl)

Cpx + Opx + Spl + melt = Ol + Gt

This pargasite breakdown model may involve geological processes as below: (1) arc-type peridotite were metasomatized by small amount of fluid/melt originated from underlying asthenosphere to form small mode of HFSE-rich pargasite; (2) paragasite then melted due to prograde metamorphism caused by subduction to produce HFSE-rich clinopyroxenes and melts trapped by garnets, which grew simultaneously; (3) diamond bearing zircon crystallized from these melts as pressure increased to return to solidus.